Progenitor diversity is generated through a combination of cell-intrinsic and environmental mechanisms. Our work focuses on the role of the environment in controlling a transition in progenitors that occurs during late embryonic development in the cerebral cortex. As a result, a second progenitor population is generated which resides in a second germinal layer, the subventricular zone (SVZ). The transition from early/ventricular zone (VZ) progenitors to late/SVZ progenitors involves changes in phenotypic potential and in morphology. We have used functional assays in explant cultures of rodent cortex in combination with retroviral transduction in vitro and in vivo to identify several environmental signals that contribute to this transition: BMP4, FGF2, Shh, and Wnts 7a and 7b. These signals also have other functions in cortical progenitors, but our data indicate that their effect on progenitor maturation reflects a distinct threshold. Progenitors within the SVZ are heterogeneous, and we have focused on a subset of late/SVZ progenitors distinguished by expression of a high level of EGFRs. These cells are of particular interest for several reasons. The EGFR-hi subset is heterogeneous and includes multipotent stem cells. EGFR-hi cells have also been suggested as a source of gliomas. Understanding the mechanisms that control their development will therefore provide information that can be applied to both regenerative medicine and cancer biology. Our previous work generated a model that suggests that a balance between antagonistic extrinsic signals controls the development of the EGFR-hi subset of late/SVZ progenitors by establishing appropriate signaling thresholds. We will test this model in vivo in aim 1 using mutant mice in which the expression of components of candidate signaling pathways has been reduced or eliminated. We will focus on the EGFR-hi subset, but extend analysis to the SVZ generally, examining the impact of candidate signals on phenotypic and morphological changes associated with the VZ-SVZ transition. The contributions of candidate signals to the generation of diversity within the SVZ, particularly within the EGFR-hi subset, will be addressed in aim 2. Our previous work identified the serine-threonine kinase Akt-1 as a possible intracellular mediator of FGF2 on progenitor maturation and on the self-renewal of multipotent progenitors. The targets of Akt- 1 that mediate maturation and/or self-renewal will be addressed in aim 3.